Have 99 aurora and its saying add oil but the dipstick reads full, have to change it anyway it sat for half a year because dash and running lamps wouldn't come on now theu do.has a gasket leak but dip

After sitting that long change the oil ,filter etc.. .Fill your tank with the Premium unleaded & add a fuel solvent that takes care of moisture , fuel tanks sitting that long build up with moisture ..when you finally start the car if you see gray or white smoke from the exhaust its the water burning off ..one other thing check your tires to see if they dry-rotted ..